In this study, 111 patients, discharged from the hospital after prosthetic heart valve replacement, were followed for evidence of embolization. Ninety-three patients received anticoagulant therapy and five had embolic episodes (5%). In contrast, systemic emboli developed in eight of 18 patients (44%) who were not given anticoagulants. Documented hemorrhagic complications occurred nine times and there were two other suspicious incidents. Prothrombin times during the occurrence of these complications were known in ten instances and were prolonged above therapeutic levels in nine of these cases. There have been eleven late deaths (10%). Precise anticoagulation appears to be of value in reducing thromboembolism from prosthetic valves. However, careful control is necessary to minimize hemorrhagic complications.